PURPOSE: To determine prospectively the diffusibility of water in normal lumbar disks in adults by using the mean apparent diffusion coefficient (ADC) and to determine if a relationship exists between disk ADC and magnetic resonance (MR) findings of disk degeneration. MATERIALS AND METHODS: The study was approved by the Institutional Review Board, and all participants gave written informed consent prior to enrollment. Diffusion-weighted MR imaging of the lumbar spine was performed in 39 patients (all men; mean age, 53 years) and five volunteers (all men; mean age, 31 years). ADC values were recorded for each disk. All disks were visually graded by two independent observers as being normal or as showing at least one of three MR findings of degeneration on sagittal T2-weighted images. Mean ADC values of normal disks were compared with those of degenerated disks and were correlated with age and anatomic location. Data were analyzed by using Kendall correlation statistics, Mantel-Haenszel statistics, and a paired two-tailed Student t test. RESULTS: The mean ADC value was 2.27 x 10(-3) mm(2)/sec +/- 0.58 (+/- standard deviation) in normal disks and 2.06 x 10(-3) mm(2)/sec +/- 0.47 in abnormal disks (9% reduction, P = .006). A statistically significant dependence of lumbar disk ADC on anatomic location was reported (analysis of variance, P < .001), with lower ADC values seen in more caudal disks. There was no association between age and mean disk ADC. CONCLUSION: A statistically significant decrease was seen in the ADC values of degenerated lumbar disks when compared with ADC values of normal disks. More caudal disks, even when normal, showed lower ADC values than more cephalic disks.